Dear Seniors:
When I want to make a Time-History analysis or a Response Spectrum analysis is necessary to make a P-Delta analysis to include the second-order effect due the deformation of the structure produced by horizontal forces. STAAD is capable to make a P-Delta effect without any problem, but in its TECHNICAL REFERENCE says:
“A PDELTA ANALYSIS will correctly reflect the secondary effects of a combination of load cases only if they are defined using the REPEAT LOAD specification... Secondary effects will not be evaluated correctly for LOAD COMBINATIONS”
But the REPEAT LOAD specification says:
“RESPONSE SPECTRUM LOAD and TIME HISTORY LOAD should not be used in REPEAT LOAD. It is also not available for loads generated using the program's load generation facilities such as UBC LOAD Generation, WIND LOAD Generation, MOVING LOAD Generation, etc”
My question is:
How I can to combine several loads case including Time History or Response Spectrum load case, or any other load derivate from generation load facility, to make an appropriate P Delta analysis?
It’s necessary to model the totality of the mass of the structure in the same directions to avoid that the mass be canceled. That's why is extremely necessary to "combine" it effect in one way (positive) and in the other way (negative) with the rest of the common load case like dead, live and others. How I can do that and reach an appropriate P Delta effect analysis?
I give you an example where I've defined a seismic load case using the response spectrum analysis, you can see the error that STAAD give me back because I use response spectrum in a repeat load specification.
25. LOAD 1 LOADTYPE DEAD TITLE Dead
26. SELFWEIGHT Y -1
27. MEMBER LOAD
28. 2 UNI GY -1.5
29. LOAD 2 LOADTYPE LIVE REDUCIBLE TITLE Live
30. MEMBER LOAD
31. 2 UNI GY -2
32. 2 CON GY -2 3
33. LOAD 3 LOADTYPE SEISMIC TITLE Seismic X
34. SELFWEIGHT X 1
35. SELFWEIGHT Y 1
36. MEMBER LOAD
37. 2 UNI GX 1.5
38. 2 UNI GY 1.5
39. 2 UNI GX 2
40. 2 UNI GY 2
41. 2 CON GX 2 3
42. 2 CON GY 2 3
43. SPECTRUM CQC IBC 2006 X 1 ACC DAMP 0.05 LIN
44. ZIP 92887 SITE CLASS B TL 8.000
45. LOAD 4 LOADTYPE NONE TITLE Repeat Load
46. REPEAT LOAD
47. 1 1.0 2 1.0 3 1.0 **ERROR- CASE BEING FACTORED IS DYNAMIC. CASE= 3
48. PDELTA KG 15 ANALYSIS SMALLDELTA PRINT MODE SHAPES
A better choice of input is as follows:
Ensure that you have requested enough modes. The default 6 is often not enough.
CUT OFF MODES 20
LOAD 1 LOADTYPE DEAD TITLE Dead SELFWEIGHT Y -1 MEMBER LOAD 2 UNI GY -1.5
LOAD 2 LOADTYPE LIVE REDUCIBLE TITLE Live MEMBER LOAD 2 UNI GY -2 2 CON GY -2 3
Combine the static cases for PDELTA. For PDELTA the cases with both horizontal and vertical forces will have more of the Pdelta effect.
LOAD 4 LOADTYPE NONE TITLE Repeat Load REPEAT LOAD 1 1.0 2 1.0
To have the PDELTA effect in dynamic cases, place the case just before the PDELTA KG Analysis command. The dynamics will include the Kg matrix from the prior case (case 4). | [K+Kg] - ω2 [M] | = 0
LOAD 3 LOADTYPE SEISMIC TITLE Seismic X SELFWEIGHT X 1 SELFWEIGHT Y 1 MEMBER LOAD 2 UNI GX 1.5 2 UNI GY 1.5 2 UNI GX 2 2 UNI GY 2 2 CON GX 2 3 2 CON GY 2 3 SPECTRUM CQC IBC 2006 X 1 ACC DAMP 0.05 LIN ZIP 92887 SITE CLASS B TL 8.000
Combine the static and dynamic by using the LOAD COMBINATION command. Create two cases: static + dynamic and static - dynamic since RSA dynamic results can be + or - . If there are 2 or more dynamic cases, you should combine those dynamic cases with SRSS option in LOAD COMBINATION.
LOAD COMBINATION 5 LOADTYPE NONE TITLE LOAD COMB 4 1.0 3 1.0
LOAD COMBINATION 6 LOADTYPE NONE TITLE LOAD COMB 4 1.0 3 -1.0
The PDELTA KG command does not need iteration value or smalldelta. They are included.
PDELTA KG ANALYSIS PRINT MODE SHAPES
Answer Verified By: Sye
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Hello Sye,
Thank you for the quick reply. Therefore the changes would be:
1. Move the PDELTA KG ANALYSIS and CHANGE from after load case 121 to a location after the load comb 17:
LOAD COMB 17 1.0*(SW+DL)-1.0*BL
4 1.0 121 -1.0
*
PDELTA KG ANALYSIS
CHANGE
PERFORM ANALYSIS *Should I still keep this command?
2. Remove the PERFORM ANALYSIS PRINT ALL command from within the design block:
I am using a SELECT command on another identical model. I believe I should apply a PERFORM ANALYSIS command after the SELECT command so that Staad updates the results for the new sections. Does this contradict your advice to remove the PERFORM ANALYSIS command from the design block?
It would look like this:
. . .
DFF 65 MEMB 696 TO 703DFF 65 MEMB 706 TO 709 718 719 732 733DFF 65 MEMB 536 TO 583 784 TO 795*RATIO 1.3 ALLRATIO 0.9 MEMB 536 TO 583 696 TO 703 706 TO 709 718 719 732 733 784 TO 795*
*** End of PARAMETER 1 - design check of the current sections
SELECT MEMB 1 TO 99 101 TO 107 109 TO 115 117 TO 138 166 TO 192 200 TO 233 -241 TO 260 268 TO 335 358 TO 360 363 TO 365 368 TO 370 373 TO 375 -378 TO 380 383 TO 385 396 TO 410 421 TO 435 446 TO 460 471 TO 485 -496 TO 510 521 TO 535 584 TO 695 782 783GROUP AX MEMB 1 TO 84 GROUP AX MEMB 85 TO 92 117 TO 138 322 TO 335 782 783 GROUP AX MEMB _COL_INT GROUP AX MEMB _RAFT_EXT GROUP AX MEMB _RAFT_INT GROUP AX MEMB _TIE_B_FRONT_R GROUP AX MEMB _TIE_B_SIDE GROUP AX MEMB 696 TO 703 GROUP AX MEMB 706 TO 709 718 719 732 733 GROUP AX MEMB _ROOF-BEAMS GROUP AX MEMB 536 TO 583 784 TO 795 *LOAD LIST 14 TO 17*
*PERFORM ANALYSIS PRINT ALL
*** Start of PARAMETER 2 - design check of the sections picked by SELECT command
**DEFINE ENVELOPE14 TO 17 ENVELOPE 1 TYPE STRESSEND DEFINE ENVELOPELOAD LIST 14 TO 17*PARAMETER 2CODE AISC UNIFIEDMETHOD ASD